This invention relates to studying the effects of flow fields imposed on a material during processing.
The engineering properties of a material are determined by the meso-, micro-, and nano-structure of the material. These features, in turn, are extremely sensitive to the processing history of the material, including the temperatures and flow fields to which the material was exposed during processing. For example, in typical commercial polymer processing operations such as extrusion molding, film blowing, or fiber spinning, a polymer melt is subjected to intense flow fields (shear, elongational, or a combination thereof), thereby distorting the melt. In the case of semi-crystalline polymers, imposing flow fields on the melt accelerates the rate of crystallization, and can result in the formation of crystallites oriented in the flow direction, both of which affect the morphology and properties of the resulting material. In the case of amorphous polymers, imposing flow fields can result in a "frozen-in" orientation that modifies the mechanical and optical properties of the material. Similarly, flow fields imposed during processing can alter the microstructure of polymer blends, filled polymers, and other materials.
Optimizing the engineering properties of materials requires an understanding of the effects of flow fields imposed during processing to permit a rational choice of suitable materials and processing conditions for a particular application. The work of Janeschitz-Kriegl et al., reported in Int. Polym. Process, 8, 236 (1993); Int. Polym. Process, 10, 243 (1995); Rheol. Acta 35, 127 (1996); and Int. Polym. Process, 12, 72 (1997), represents one approach towards understanding the underlying physics of polymer melt crystallization. Janeschitz-Kriegl subjected a subcooled polymer melt to brief, intense shearing at shear rates similar to those experienced in typical polymer processing operations by driving the polymer through a slit under high pressure generated by an extruder, after which the polymer was allowed to crystallize. The progress of crystallization was tracked using a rotating polarizer setup that monitored birefringence. The resulting data provided information regarding the relationship between crystallization time and both wall shear rate and shearing time.
The apparatus used in the Janeschitz-Kriegl studies suffers from two disadvantages that limit its commercial utility. First, using an extruder requires the use of large amounts of polymer samples, thereby limiting the number and type of polymers that can be studied. In addition, the rotating polarizer set-up limits the time resolution of the data, making it difficult to monitor the deformation of the melt during short shearing times.
Capillary rheometers have also been used to study the flow properties of polymeric materials. The rheometer is operated by forcing polymer through capillaries of varying lengths using pneumatic or hydraulic pressure, a screw feed, or a dead weight.
Capillary rheometers are typically used to study isothermal, steady state, stress-strain relationships. However, they are not well-suited for generating well-defined transient deformations and recording structure development in real time while the polymer is under the influence of the flow field.